Portable apps will not really provide security against an infected public computer. They may actually reduce your security on public computers for the following reasons:

USB thumb drives easily get infected by malware. You can use a write-protect switch to prevent bad programs from infecting your thumb drive or portable hard drive, but then you can't save your settings to the device and you might as well just download a fresh copy of the apps to each computer that you use.

USB thumb drives can get copied over the network. I've seen this happen before. If your passwords are stored on your thumb drive in a program like Portable Firefox, then you could lose all of your passwords at once. You can set a master password, but that can be cracked at the lesiure of the criminal who has secretly copied your passwords. This is especially easy if you have used a master password for your Portable Firefox password list, becuase someone could use a keylogger to get your master password and secretly copy your entire thumb drive at the same time.